Original Article
In Vitro Evaluation of the Effect of Core Thickness and Fabrication Stages on the Marginal Accuracy of an All-Ceramic System
F. Farid1, H. Hajimiragha2, R. Jelodar3, A. S. Mostafavi4, H. Nokhbatolfoghahaie5
1Dental Research Center and Department of Prosthodontics, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran
2Department of Prosthodontics and Dental Research Center, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran
3Department of Orthodontics, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran
4Department of Prosthodontics, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran
5Dentist, Laser Research Center of Dentistry (LRCD), School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran
Corresponding author:
H. Hajmiragha, Department of Prosthodontics, School of Den- tistry, Tehran University of Medical Sciences, Tehran, Iran [email protected]
Received: 12 March 2012 Accepted: 20 June 2012
Abstract
Objective: To evaluate the influence of core thickness and fabrication stages on the marginal accuracy of IPS e.max Press crowns.
Materials and Methods: Twenty IPS e.max Press crowns, 1.5mm thick, were fabricated on metal dies. The crowns had two different core thicknesses, 0.8mm for group A and 1mm for group B, ten for each group. Marginal gap was meas- ured after each stage of core fabrication, veneering and glaze firing. The speci- mens were not cemented and the measurements were made at four points on met- al dies using a stereomicroscope (×120). Data were analyzed by SPSS software and independent t-test.
Results: Mean marginal gaps measured after each stage for group A were 13.5 (±1.4) µm, 33.9 (±2.3) µm and 40.5(±1.7) µm, and for group B these figures were 14.9(±2.0) µm, 35.5(±2.2) µm and 41.3(±2.0) µm. There were no statistically sig- nificant differences in marginal gap values between the two groups (p>0.1). Sig- nificant increase in gap was observed after the veneering stage in both groups (p<0.05). After glazing, no significant increase in gap was detected.
Conclusion: IPS e.max Press crowns have an acceptable marginal fit. Increasing thickness of core does not increase marginal fitness.
Key Words: Marginal Fitness; Marginal Gap; IPS e.max Press
Journal of Dentistry, Tehran University of Medical Sciences, Tehran, Iran (2012; Vol. 9, No. 3)
INTRODUCTION
Marginal integrity is a key factor in the suc- cess of restorations. In cemented restorations it means close adaptation of the prosthesis mar- gin to the finishing line of prepared tooth.
Gaps in margin render the tooth susceptible to caries, pulpitis and periodontal disease [1-3].
Improving adaptation can increase longevity of treatment and the success rate.
All-ceramic restorations are in fast progression in properties and usage. In situations with high esthetic demands, these materials may be the material of choice. Although adhesive resin cements are recommended for most of all-
ceramics [1], and these luting agents have very low solubility [4], fitness is still important.
The incidence of gingival inflammation i creases around clinically deficient restorations, particularly those with rough surfaces, subgi gival finish lines or poor marginal adaptation [1]. Factors affecting adaptation in all
restorations may be categorized into those i fluencing before cementation [5
influencing after cementation
changes in marginal fitness after thermal or mechanical loading in the mouth
These factors have influence before cement tion: ceramic system (polycrystalline based ceramics, glass-ceramics, glass
ramics and feldspathics) [1,18]
turing technique [1,2,6,19] (sintering, slip casting, heat pressing, casting, CAD/CAM), fabrication stages,[19-21] finish lines
core and veneering layer thermal mismatch [23].
In techniques that a strong core is veneered by a layer ceramic, thermal mismatch between
Fig1. Left, metal die used for veneering and right, template with certain inner diameter for core wax-up
, and these luting agents have very fitness is still important.
The incidence of gingival inflammation in- clinically deficient restorations, particularly those with rough surfaces, subgin- gival finish lines or poor marginal adaptation
Factors affecting adaptation in all-ceramic restorations may be categorized into those in-
5-7] and those [8-12] and changes in marginal fitness after thermal or mechanical loading in the mouth [13-17]
These factors have influence before cementa- tion: ceramic system (polycrystalline based
ceramics, glass infiltrated ce- the manufac- (sintering, slip- casting, heat pressing, casting, CAD/CAM),
finish lines [6, 7, 22]
core and veneering layer thermal mismatch es that a strong core is veneered by a layer ceramic, thermal mismatch between
two materials can cause deflections during the fabrication process[23]. In the study of Caste lani, thermal distortion of all
was compared with metal ceramic [24]. The materials exhibited different re ponses. The metal ceramic crowns stabilized after an initial loss of marginal precision, and the all-ceramic crowns continued to distort with each firing cycle. Balkaya et al. reported that the addition of porcelain to the copings caused a significant change in the marginal fit of the crowns made from conventional In Ceram, copy-milled In-Ceram and copy feldspathic crowns [19]. Sulaiman et al. co pared marginal fit of In-Ceram, Procera, and IPS Empress crowns [20].
ferences were detected among the various stages of crown fabrication. Both studies found significantly larger marginal discrepa cies in the facial and lingual surfaces than m sial and distal. Greater bulk of porcelain was explained as the probable reason for higher discrepancy in these surfaces
et al. reported, ceramic firing did not signif cantly affect either marginal or internal adapt tion of the Cercon, a ZrO2
difference between studies is possibly a result of the greater strength of ZrO
The purpose of the present study was to inve tigate the influence of core and veneer thic ness on marginal adaptation of IPS e.max Press (Ivoclar Vivadent, Schaan, Liechten tein) crowns, during different stages of core fabrication, veneering and glazing. IPS e.max Press is a heat pressed lithium disilicate glass ceramic, which is veneered by IPS e.max C ram, a nano-fluorapatite glass
material is the last generation of pressed c ramics from Ivoclar Vivadent Company, with improved strength and optical properties. Its flexural strength is 400MPa (50MPa greater than IPS Eris). It is indicated for anterior and posterior single crowns, for bridges
second premolars and implant superstructures.
The null hypothesis was that
die used for veneering and r diameter used
two materials can cause deflections during the In the study of Castel- lani, thermal distortion of all-ceramic crowns
metal ceramic crowns The materials exhibited different res- ponses. The metal ceramic crowns stabilized after an initial loss of marginal precision, and ceramic crowns continued to distort with each firing cycle. Balkaya et al. reported porcelain to the copings caused a significant change in the marginal fit of the crowns made from conventional In-
Ceram and copy-milled Sulaiman et al. com-
Ceram, Procera, and No significant dif- ferences were detected among the various stages of crown fabrication. Both studies found significantly larger marginal discrepan- cies in the facial and lingual surfaces than me- sial and distal. Greater bulk of porcelain was
xplained as the probable reason for higher discrepancy in these surfaces [19,20]. Komine , ceramic firing did not signifi- cantly affect either marginal or internal adapta-
2 ceramic [25]. The difference between studies is possibly a result of the greater strength of ZrO2 core [25].
The purpose of the present study was to inves- tigate the influence of core and veneer thick- ness on marginal adaptation of IPS e.max
nt, Schaan, Liechtens- tein) crowns, during different stages of core fabrication, veneering and glazing. IPS e.max Press is a heat pressed lithium disilicate glass- ceramic, which is veneered by IPS e.max Ce- fluorapatite glass-ceramic. This l is the last generation of pressed ce- ramics from Ivoclar Vivadent Company, with improved strength and optical properties. Its flexural strength is 400MPa (50MPa greater than IPS Eris). It is indicated for anterior and posterior single crowns, for bridges anterior to second premolars and implant superstructures.
that increasing the
core thickness has no effect on marginal accu- racy of this ceramic.
MATERIALS AND METHODS
Three stainless steel dies were machined simu- lating a premolar, prepared for full coverage, with 7.5 mm height, 7mm diameter, and 6o convergence angle (Fig 1). Finish line was rounded shoulder, with different width for each die. The minimum framework thickness recommended by Ivoclar Vivadent was 0.8mm and the maximum thickness for the veneering layer was 0.7mm. So cores were fabricated on 0.8mm (group A) and 1mm shoulder dies (group B), ten specimens for each group. The 1.5mm shoulder die was used for veneering cores to final crowns. Special metal templates were machined to insure similar thickness for cores in each group and final crowns (Fig 1).
A groove, 4mm long and 0.5mm deep was in- serted on dies for repeatable seating of sam- ples and preventing their rotation. Depressions were made on four sides of dies 0.5mm below margins, pointing identical positions for mea- surements. Die spacer was not used.
To fabricate cores, templates were placed on related dies and wax was injected in between.
During removing wax patterns from metal dies, any incomplete one, due to attaching to metal was remade. Wax patterns were re- moved, sprued and invested in IPS PressVest speed investment material (Ivoclar Vivadent,
Schaan, Liechtenstein). Preheating of invest- ment rings was done at 800oc. Then rings were transferred to Ivoclar EP600 press furnace (Ivoclar Vivadent, Schaan, Liechtenstein) and press filled with IPS e.max Press, [K20768 (Ivoclar Vivadent, Schaan, Liechtenstein)] HO ingots at 920oc. Then the cores were divested, sandblasted with glass polishing beads at 2 bar (30 psi) pressure and cleaned by Invex (Ivoclar Vivadent, Schaan, Liechtenstein) in ultrasonic bath to remove the reaction layer formed on them. Any incomplete cast core was discarded.
Each core was seated on the main die (1.5 shoulder) and the assembly was placed in a device designed to keep them together and prevent their movement (Fig 2). One end of the device was not fixed and could apply pres- sure on the core, to insure close adaptation of core to die. Marginal fit was examined at ×120 magnification by a stereomicroscope (SZX12 Olympus, Japan) at four predetermined points.
According to Holmes definition for marginal gap [26], the distance between crown margin and the edge of die was measured. After this stage, all cores were reinserted on the 1.5mm shoulder die and IPS e.max Ceram [K3116 powder, K43221 liquid (Ivoclar Vivadent, Schaan, Liechtenstein)] as the veneering layer was applied on them using a template to insure uniform thickness. The marginal 0.5mm of cores was not veneered. Each veneered crown was inserted on the main die and again the as- sembly was placed in the device and the fit- ness was examined under the stereomicros- cope. The process was repeated after glazing [H29987]. All laboratory procedures were done by one technician and all examinations were done by one of the investigators. Data were analyzed by SPSS software and indepen- dent t-test.
RESULT
Mean marginal gaps measured after core fabri- cation, veneer firing and glaze were 13.5 (±1.4) µm, 33.9 (±2.3) µm and 40.5 (±1.7)
Fig 2. A crown and related die in the holding device
µm, respectively for group A and 14.9 (±2.0) µm, 35.5 (±2.2) µm and 41.3 (±2.0) µm, r spectively for group B. There were no signif cant differences in marginal gap values b tween the two groups (p>0.1). Significant i crease in gap was observed after veneering stage in both groups (p<0.05), but no signif cant increase was detected after glazing in ther of the two groups. The results are shown in figure 4.
DISCUSSION
This study was designed to evaluate the infl ence of core and veneer thicknesses on ma ginal adaptation of IPS e.max Press crowns during different stages of fabrication. The r sults showed 0.2 mm increase in core thic ness had no effect on the marginal accuracy of crowns and the null hypothesis was confirmed.
Addition of the veneering layer significantly decreased marginal adaptation of the crowns (p<0.05).
In this study, metal dies were used to prevent wear of dies during fabrication processes and measurements [19]. Finish line was rounded shoulder that is suggested for all
coverage crowns [27].
Core thicknesses were 0.8 and 1mm, because the least thickness recommended by the man facturer was 0.8 mm. Die spacer was not used because crowns were not cemented;
Fig 3. Stereomicroscopic view of marginal gap at core stage, (a) group A, (b) group B
a
µm, respectively for group A and 14.9 (±2.0) (±2.0) µm, re- spectively for group B. There were no signifi- cant differences in marginal gap values be- tween the two groups (p>0.1). Significant in- crease in gap was observed after veneering stage in both groups (p<0.05), but no signifi-
ed after glazing in ei- of the two groups. The results are shown
This study was designed to evaluate the influ- ence of core and veneer thicknesses on mar- ginal adaptation of IPS e.max Press crowns
fabrication. The re- mm increase in core thick- ness had no effect on the marginal accuracy of crowns and the null hypothesis was confirmed.
Addition of the veneering layer significantly decreased marginal adaptation of the crowns this study, metal dies were used to prevent wear of dies during fabrication processes and Finish line was rounded shoulder that is suggested for all-ceramic full Core thicknesses were 0.8 and 1mm, because thickness recommended by the manu-
mm. Die spacer was not used cemented; further
more, die spacer application might complicate uniform thickness of the samples
affect crown fitness [19]. The marginal 0.5mm of cores was not veneered, because veneer m terial on the margin could lead to misleading results [25].
The methods used for measurement of the marginal fit include: 1) direct view of the crown on a die, 2) cross-sectional view, 3) i pression replica technique, and 4) clinical e amination [19]. The last three methods are suitable for measuring gaps after cementation and after loading in the mouth. In this study, uncemented, nonsectioned specimens were examined by direct view under the stereom croscope. To reposition them repeatedly, a groove was inserted on dies. Grooves and d pressions helped all examinations to be done in identical points. Because of symmetrical contour in the specimens, only four me ment points per specimen were considered.
The sample size of ten for each test group was for compensating minor variations in the fabr cation process.
In the present study, the mean marginal gap of IPS e.max Press crowns was 41µm. Stappert et al. studied marginal adaptation of different types of all-ceramic partial coverage restor tions after exposure to an artificial mouth. The result of the present study is in agreement with values reported for four cusps reduced group
×120
tereomicroscopic view of marginal gap at core stage, (a) group A, (b) group B
b
more, die spacer application might complicate uniform thickness of the samples [24] and may The marginal 0.5mm of cores was not veneered, because veneer ma- terial on the margin could lead to misleading The methods used for measurement of the marginal fit include: 1) direct view of the sectional view, 3) im- pression replica technique, and 4) clinical ex- The last three methods are suitable for measuring gaps after cementation and after loading in the mouth. In this study, uncemented, nonsectioned specimens were examined by direct view under the stereomi- croscope. To reposition them repeatedly, a groove was inserted on dies. Grooves and de- pressions helped all examinations to be done in identical points. Because of symmetrical contour in the specimens, only four measure- ment points per specimen were considered.
The sample size of ten for each test group was for compensating minor variations in the fabri- In the present study, the mean marginal gap of IPS e.max Press crowns was 41µm. Stappert et al. studied marginal adaptation of different ceramic partial coverage restora- tions after exposure to an artificial mouth. The result of the present study is in agreement with values reported for four cusps reduced group
×120
(50µm) before cementation [14]. Conrad et al.
reviewed current literature for in vivo and in vitro marginal discrepancy evaluations. The value reported for IPS Empress 2 was 44 µm [1]. Balkaya et al. reported that the labial and palatal surfaces of the conventional and copy milled In-Ceram crowns exhibited significant- ly larger marginal discrepancies than the other surfaces [19]. The results are consistent with the findings of Sulaiman et al. for Procera, IPS Empress and In-Ceram [20]. They explained the result by shrinkage of porcelain toward the greatest mass. Their specimens were made on dies simulating prepared maxillary central in- cisor, with a wider shoulder and as a result, a greater mass of porcelain at the labial and pa- latal surfaces compared to the mesial and dis- tal [19, 20].
In the present study, there was no variation in shoulder width and veneer thickness inside one group. This may explain the difference of
results between the studies. Changing core and veneering layer thickness in different groups did not influence the fitness either.
In this study, the marginal gap increased in every stage of fabrication. Significant increase (p<0.05) was observed after firing veneering layer. This result agrees with what Balkaya et al. found in their study. They indicated that addition of porcelain to the copings caused a significant change in the marginal fit of the crowns [19], but Sulaiman et al. found no sig- nificant differences among the various stages of crown fabrication [20].
Low shrinkage of heat pressed cores may ex- plain the difference. For heat pressed ceramics that use lost wax technique, 0.4% shrinkage of wax pattern and 0.2% shrinkage of ceramic coping can be compensated by 0.3% setting and 0.2% thermal expansion of investment material [20]. Because of inherent properties, large gaps are not expected in pressed copings
10
10 10
10 10
10 N =
95% CI
50
40
30
20
10
Glaze Group A Group B
Veneer
Marginal gap (µ m) 95% CL
Fig 4. Comparison of Marginal Gaps between Two Groups According to Fabrication Stages N=
Core Glaze
[28]. In the present study, the mean marginal gap of copings was 14.2µm. After veneer fir- ing there was a significant increase in gap.
This is different from what Komine et al.
found in their study [25].
Their comparison between ZrO2 copings and crowns demonstrated that ceramic firing did not significantly affect marginal adaptation.
They explained that the strength of ZrO2 cop- ings prevent them from being affected by por- celain firing procedure. Lithium disilicate glass-ceramics are not as strong as ZrO2 cop- ings (400MPa compared with 900MPa). In- crease in marginal discrepancy after veneer firing is a probability.
Glaze had no significant effect on gap, similar to w h a t Balkaya [19] et al. reported. The values for marginal gaps in both groups were in a clinically accepted range proposed by Christensen [29], McLean and Fraunhofer [30].
CONCLUSION
IPS e.max Press crowns have an acceptable marginal fit.
The comparison between the present study and previous studies indicate that variation in ve- neer thickness of one crown is more influential on marginal accuracy than uniform change in veneer or core thickness.
ACKNOWLEDGMENTS
This research was presented as graduation the- sis in October 2009 and supported by Tehran University of Medical Sciences research grant (4584-69-04-85). Ivoclar Vivadent Company provided the materials. The research was awarded in the 9th annual dental symposium.
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